The filter stage around U1 is the actual circuit. It is for a woofer response EQ.

However, stage 4 / power amplifier is only for illustration. The actual amplifier is Randy Slone's Optimos with my own component upgrades and elimination of protection circuits and input caps. It sounds fantastic. I could increase its input impedance but I really worry about such changes would have impacts on other areas. Slone's amp is perfect and I don't want to mess it up.

Adrome00,

Thank you for your advice and I will look into it in greater details with a print out copy on my way home. I am now finishing work and about to leave. Thanks again.

Your concerns about impedances in the simplified circuit were what motivated my suggestion regarding a BUF634. It presents an input impedance of many M-ohms (virtually infinite) which optimizes the environment of the filter circuit. At the same time the output can deliver more current than you are likely to need --thus limiting the range of the pot is not necessary. (The diagram you showed has the pot connected as a variable resistor which based on my personal experience is a dissapointing volume control.)

A filter circuit where the OPamp is pushed so far as to be unable to drive a 10-22k pot is simply a badly designed filter. From that standpoint you do not need to buffer the output of the filter before going to the volume pot.
If you are using a fully DC connection to the power amp, unless the input is FET based or bias current compensated (most dual differential amps are), you will need either an input cap or a buffer after the pot to prevent ofset changes. Anything over a few uA DC through the pot wiper is also a concern.
It is indeed likely that for a given quality ofsound, an OPamp buffer or a dedicated buffer chip will indeed be cheaper than a coupling cap. In your case you would be looking at something on the order of >20uF to prevent LF distortion and these are not cheap. If you use a buffer, be sure it has negligible input current (one of the simplest ways to ensure this is to use a FET input OPamp/buffer).

I would like to know more about how a trimpot would degrade the sound. I thought at the worst its noise is higher comparing to a metal film resistor but could not think of a way it could introduce distortions. Perhaps the tolerance with regards to temperature rise? or its higher inductance? but the inductance would still be so low that it shouldn't have any effect within the audio frequencies. I have done A/B tests on caps used in XOs and caps do make a profound difference in sound quality. I have also compared some cheap non-inductive wirewound with some metalised silicon resistors in XOs and both sounded respectful. I have not compared resistors with trimpots at line level. At the end of the day, when the system is tuned and finalised, I could always replace the trimpots with fixed value resistors. Your post aroused my concerns because I am using quite a few trimpots in my various circuits. If they are no good, what alternatives do you recommend?

Thank all of you for your input. I have checked the circuit and found that I can certainly insert P1 before U1, in that case U2 and U3 in my first diagram can be deleted. This is a good simplification of the original circuit. The sound will certainly be clearer by removing the input cap and 2 opamps in the signal path while maintaining the ability for attenuation without impedance matching problems. Succeeded!

How silly I was! I had not thought of that because it was somebody's design so I was only thinking about interstaging instead of looking into the details of the circuits. The active circuits I am designing / modifying include a LR-4 filter for my hybrid active WWMTMWW tower speaker and all the filters for John K's NaO dipole / U-frame speaker (I got his earlier free plan but not his latest board / circuit plan).

Now that leads me to have some new questions. More to follow when I can get a moment to draw the diagrams.

Since your XO/EQ output is to be level matched surely the gain should be placed there, and, since you only require a small 'trimming' range (say 6DB) why not trim R2 the input leg of the feedback network around U1 if it's accessible. Then the low Zout of U1 can drive the 10K input of your power amplifier directly.

The previous stage is a valve preamp with a gain of 5.6 and output impedance of 900R connected to a typical CD player at the input. The next stage are 2 power amplifiers with a gain of 22(?) and input impedance of 10k.

At which point should the attenuation trimpot P1 be inserted so that noise and distortions of the OPA314s be minimized and these OPA314 operate optimally?

I have looked up the datasheet of OPA314 but with my limited knowledge I simply don't know what is the averaged musical signal input amplitude therefore can't work out what are the optimal drives for them. The trimpot is necessary so that SPL adjustments between woofer and HF drivers can be made.

Your help will be much appreciated.
Regards,
Bill

amplifierguru,
I have typed up this one then saw your post. I am posting it anyway but it seems your post happens to be related to this new post. I will digest it. Thanks.

I didn't mean to imply it would be a source of distortion other than in the same fasion as any other resistor. I meant that when used as a variable resistor in series rather than as a variable voltage devider that the range of attentuation will be limited. Maybe that will be sufficient or maybe not, but you usually don't get a full volume range all the way down to "off". The only way to get more attenuation is to make the pot value larger.

The other thing is that on nearly every (well, a great many, anyway) opamp data sheet, reducing the load shifts the THD+N line upwards. So if the opamp is run direcctly into a pot, the performace of the opamp changes when you change the pot setting. It may very well be the case that the dfference will not be audible or not enough that you notice without focusing hard on it. I.e., good enough for most people most of the time. Your post didn't sound like you were from the "good enough" school of audiophilia!

I think amplifierguru has answered my last question. Since the adjustment is made to individual channels of course the pot has to be after stage 3. I think it makes sense to place it before (6) (U1 in my previous diagrams).

Amplifierguru,

You are right that another way to do it is to make R2 to be a pot. This has an added advantage that the pot can be used not only for attenuation but also for gain as well. However, there is a slight complication there. The feedback impedance is just a bit below 2k (sorry I have not given the values in the schematic). When attenuated the value may need to increase to above 2K. According to the OPA314 datasheet, in that case the input impedance of the non-inventing input must be matched or distortion will rise. It will work if I add another pot to shunt the non-inverting input to ground to match the impedance of the feedback (R1 || R2), I guess. Please correct me if I am wrong.

My remaining questions are:

(1) referring to the above schematic, given that it is a typical CDP driving a preamp with a gain of 5.6 which drives this active circuit that consists of mainly unity gain buffers, what are the ranges of values for resistor R1, R2, R3 that will drive the OPA314 optimally without over-driving or under-driving hence reducing noise and distortions?

(2) I previously suggested attenuation because I wanted to reduce complexity. If I want gain, R2 and R3 need to be changed to a pot. In that case, should I install R1 as a pot and set R1 = R2 || R3, providing that the previous stage has low output impedance?

If you run a school of "good enough" of audiophilia I will be your first student to enrol. I have wasted 12 years on playing with commercial first and second hand HiFi gears and only last year started getting into this crazy hobby of Audio DIY. Fortunately the system I have built and the speaker I have designed sounds better than some 6 digit mega bug systems according to my tube fanatic friends. With helps from you guys I am learning electronics and fine-tunning my system. I am open minded and am not sure if I am in the process of being educated and transformed into a worshiper of BG caps, shottkey diodes, all silver and gold wirings, cat 5 cables, etc. My speaker cable, an audio nut once said it was very impressive, was made from the cheapest electric cable and it is being the only speaker cable I am using. The Audioquest $600 speaker cable somebody gave to me for free is sitting inside a box and has never been used because it is not long enough.